This invention relates to monolithic integrated circuits and particularly to a monolithic composite integrated circuit of a field-effect semiconductor device and a Schottky diode. The field-effect semiconductor device may take the form of a metal-semiconductor field-effect transistor (MESFET) or high-electron-mobility transistor (HEMT) among others.
The MESFET and HEMT have both been known and used extensively which are made from semiconducting nitrides. Japanese Unexamined Patent Publication No. 2005-158889 is hereby cited as dealing with these kinds of semiconductor devices.
Let us more closely study the typical prior art construction of the HEMT for example. It comprises an electron transit layer of undoped GaN overlying a silicon substrate via a buffer layer, an electron supply layer of n-type AlGaN on the electron transit layer, and a set of electrodes known as a source, drain and gate on the electron supply layer. The electron transit layer and electron supply layer have a heterojunction therebetween, being made from materials with different band gaps. The piezoelectric and spontaneous depolarization of the heterojunction surfaces creates the familiar two-dimensional electron gas layer as the channel between drain and source. Current flow along this channel is under the control of the voltage bias on the gate.
One of the problems with the HEMT of the above familiar design, as well as with the MESFET or other comparable field-effect semiconductor devices, arose when these devices were connected to an inductive or capacitive load. The drain could then become less in potential than the source, resulting in the application of a reverse voltage to the semiconductor device. A known solution to this problem was the connection of a feedback, regenerative, or protective diode in parallel with the field-effect semiconductor device. This diode had its cathode coupled to the drain, and its anode to the source, of the device. The prior art insulated-gate FET (IGFET) makes use of a parasitic diode for the above purposes. The parasitic diode appears in the form of a pn junction between the body and drain regions as the source electrode of the IGFET is coupled both to the source region and to the body region surrounding the source region.
However, difficulties have been experienced in applying the parasitic diode technology of the IGFET to the HEMT, MESFET and like field-effect semiconductor devices which make use of the two-dimensional electron gas layer as the channel. Japanese Unexamined Patent Application No. 2003-229566 teaches to provide the required diode (e.g., Schottky diode) lies on that side of the source which is away from the gate, and to connect this diode to the source. The two-dimensional electron gas layer of the field-effect semiconductor device will then be interrupted by the depletion region spreading from the gate when the device is off, so that there will be no current flow between the diode and the drain.
The above difficulties have so far been circumvented by connecting a discrete diode to the field-effect semiconductor devices in question. This makeshift measure is of course objectionable for the extra installation space demanded by the discrete diode and the unnecessarily high manufacturing costs required.
Another problem taken up by the instant invention in regard to the field-effect semiconductor devices under consideration is the difficulty of making them normally off. The HEMT of the usual prior art construction above, for example, was normally on and had to be turned off using a negative power supply for causing the gate to gain a negative potential. Use of such a negative power supply made the associated circuitry unnecessary complex and expensive. The advent of normally-off HEMTs has long been awaited.
The problems and difficulties discussed above are not limited to the field-effect semiconductor devices that utilize the two-dimensional electron gas as the channel. The same discussion is largely applicable to two-dimensional hole gas counterparts of the field-effect semiconductor devices as well.